1. Field of the Present Invention
The present invention relates generally to the field of gas detectors, and, in particular, to the art of more efficiently firing, biasing and testing xe2x80x9cheated electrodexe2x80x9d halogenated refrigerant sensors using control theory to control the operation of the detector using an advanced sensing device and one or more control loops.
2. Background Art
The need for a reliable method or apparatus for detecting leaks from refrigerant systems has long been well known. A number of leak detectors have been developed to meet this need. One well-known type of leak detector makes use of a xe2x80x9cheated electrodexe2x80x9d sensing device to indicate the presence of trace quantities of halogenated refrigerants. Such sensors are generally constructed of a platinum anode/heater coil and a platinum cathode wire within the coil, and are coated with a ceramic slurry consisting of alumina and a silicate of an alkali metal such as sodium, potassium, or rubidium. The ceramic forms an electrically resistive layer between the electrodes. When heated by an electrical current passing through a first of the electrodes, an outer layer depleted of ions develops along the electrodes. When this layer is exposed to reactive gases like halogen, ions flow across the depletion zone and the conductivity of the device is increased. Thus, the presence of halogenated gases may be indicated by monitoring the current generated through the second electrode, referred to as the bias current, for a sudden increase in magnitude created by introducing the device to such gases. These sensors are commonly utilized by technicians to determine whether a refrigerant leak exists and to pinpoint its source.
For such a device to function reliably as a refrigerant sensor, the assembly must be fired (to sinter the ceramic), and also xe2x80x9cbiasedxe2x80x9d to create the ion depletion region across which ions flow in the presence of refrigerant. Typically, the firing and the biasing operations are performed separately. The firing operation takes place in a kiln at a high temperature and requires a relatively lengthy period of time. Subsequently, the fired assembly is mounted in its holder, which may be a TO-5 transistor can, and current is passed through the anode coil to heat the sensor while a bias voltage is applied between the anode coil and cathode wire. Over another relatively lengthy period of time, the depletion region is formed.
Unfortunately, prior art systems and methods have a number of significant drawbacks. First, the firing operation typically requires 3 or more hours, and the biasing operation requires up to 12 more hours, and thus collectively consume a very large amount of manufacturing time before the sensor testing for compliance to specifications may even begin. Also, the holder, such as the TO-5 transistor can, typically cannot withstand the high firing temperatures to which it must be subjected in the kiln, and thus the anode/cathode assembly must be fired separately and attached to the can afterwards, prior to biasing. Therefore, significant time and labor may have already transpired without any knowledge as to the ultimate performance (or lack there of) of the final sensor. Further, the sensor must be completely tested under operating conditions, rather than during or immediately after the biasing process, to ensure compliance to specifications, because variations in the ceramic mixture and in the construction of the sensor may have significant impact upon the final operation of that particular sensor. Finally, since the construction and the firing, biasing and testing operations require so much time, the operations must occur in large batches such that production throughput is optimized. Variations in the process could potentially produce hundreds or thousands of failures, resulting in a great deal of waste material and lost time, in addition to the time required to bring the processes back into specification.
Alternatively, some sensors are fired by passing a current through the coil to obtain temperatures sufficient to sinter the sensor. Biasing may also be accomplished by maintaining the elevated temperatures for an extended period of time. However, existing electrical heating methods involve only the crude application of a sufficient amount of heat to sinter and bias, without regard to how the temperature is raised to such temperatures, thus raising the risk of damage to the sensor during the process. No consideration is given to the amount of moisture present in the sensor as the temperature is increased. Further, such methods do not take into consideration any information about the state of the sensors being fired or biased while the heating is taking place, and thus result in significant inefficiencies in the amount of time required for the manufacture of sensors.
Thus, a need exists for an improved method of manufacturing heated electrode refrigerant sensing devices.
Briefly summarized, the present invention relates to methods and apparatuses for automatically controlling the processes of biasing and testing heated electrode refrigerant sensors for use in refrigerant leak detectors in general. Broadly defined, the present invention according to one aspect includes a method of, and apparatuses for, manufacturing a heated-electrode refrigerant sensor, having a cathode and an anode, for use in a refrigerant detector, wherein the method includes the steps of:
mounting an unbiased sensor in a manufacturing station; and while the unbiased sensor remains mounted in the manufacturing station, biasing the sensor by applying current through the sensor to electrically heat the sensor and by applying a voltage potential between the anode and the cathode, thereby generating a bias current at the cathode, and, after at least partially biasing the sensor, utilizing the bias current to electrically test the construction of the sensor.
In features of the aspect, the current and the voltage potential are continuously applied while biasing and electrically testing; the method further includes holding the temperature of the sensor essentially constant while biasing, and the temperature of the sensor while utilizing the bias current to electrically test the sensor remains essentially equivalent to the temperature of the sensor while biasing; the temperature of the sensor while utilizing the bias current to electrically test the sensor varies from the essentially constant temperature of the sensor while biasing by no more than 20 percent; the essentially constant temperature of the sensor while biasing and while utilizing the bias current to electrically test the sensor is between 900 and 1100 degrees Celsius; the sensor has a ceramic coating, and utilizing the bias current to electrically test the sensor includes testing the construction of the sensor""s ceramic coating by determining whether the magnitude of the bias current decreases to a predetermined value within a predetermined period of time; if the bias current drops to the predetermined value before the expiration of a first predetermined period of time, then an insufficient quantity of ceramic coating has been applied to the sensor or the chemical composition of the ceramic coating is imbalanced, while if the bias current does not drop to the predetermined threshold before the expiration of a second predetermined period of time, then an excessive quantity of ceramic coating has been applied to the sensor or the chemical composition of the ceramic coating is imbalanced; utilizing the bias current to electrically test the sensor occurs at least partially at the same time as biasing; utilizing the bias current to electrically test the sensor includes monitoring an output signal for noise; the monitored output signal is indicative of the temperature of the sensor; and the method further includes maintaining the magnitude of the bias current constant while the temperature signal is monitored for noise.
The present invention according to another aspect includes a method of, and apparatuses for, manufacturing a heated-electrode refrigerant sensor for use in a refrigerant detector, wherein the method includes the steps of: generating a bias current in the sensor; setting the temperature of the sensor to a bias temperature; monitoring the bias current while holding the temperature essentially constant at the bias temperature; and determining the acceptability of the sensor on the basis of the amount of time that elapses before the magnitude of the bias current decreases from an initial value to a predetermined threshold value.
In features of this aspect, setting the temperature of the sensor includes increasing the temperature to the bias temperature, and the amount of elapsed time is measured from the time that the temperature of the sensor reaches the bias temperature; if the magnitude of the bias current drops to the predetermined threshold before the expiration of a first predetermined period of time, then the sensor is rejected; the bias temperature is a second bias temperature, and the method further includes increasing the temperature of the sensor to a first bias temperature before increasing the temperature of the sensor to the second bias temperature, and the amount of elapsed time is measured from the time that the temperature of the sensor reaches the second bias temperature; the first predetermined period of time lies in the range 360 to 600 seconds; if the magnitude of the bias current does not decrease to the predetermined threshold value before the expiration of a second predetermined period of time, then the sensor is rejected; the second predetermined period of time lies in the range 900 to 3600 seconds; if the magnitude of the bias current drops to the predetermined threshold value after the expiration of a first predetermined period of time but before the expiration of a second predetermined period of time, then the sensor is acceptable; the method further includes providing an indication of the outcome of the determining step; and if the determining step results in a determination that the sensor is unacceptable, then further manufacturing operations on the sensor are discontinued.
The present invention according to another aspect includes a method of, and apparatuses for, testing a heated-electrode refrigerant sensor, the method including the steps of: generating a bias current in the sensor; generating a first signal at least partially representative of the magnitude of the bias current, the magnitude of the bias current being a first operating condition; generating a second signal at least partially representative of a second operating condition; maintaining the magnitude of the bias current at a generally constant level on the basis of the first signal; and monitoring the second signal for noise.
In features of this aspect, the steps of generating a bias current, generating first and second signals, maintaining the magnitude of the bias current, and monitoring the second signal occur while the sensor is mounted in manufacturing station; the second operating condition is a temperature of the sensor; the method further includes determining the acceptability of the sensor on the basis of the presence of noise on the second signal; if the noise which is present on the second signal exceeds a predetermined limit, then the sensor is determined to be unacceptable; the method further includes biasing the sensor, prior to the monitoring step, by heating the sensor for a first period of time; the temperature of the sensor remains continuously elevated during biasing and monitoring; the method further includes providing an indication of the quantity of noise present on the second signal to a user; the indication may be provided only upon the detection of a predetermined threshold quantity of noise on the second signal; and the amount of noise on the second signal may be regularly displayed to the user.